1. Field of the Invention
The present invention relates to a process for preparing fluorocarboxylic acids and their derivatives. More particularly, it relates to a process for preparing fluorocarboxylic acids in high purity and in high yield from the corresponding fluorocarboxylic acid fluorides. The invention relates specifically to a process for preparing C4-14, and especially C7-9, fluorocarboxylic acids, including fluoroalkylcarboxylic acids, particularly perfluoroalkylcarboxylic acids, as well as derivatives thereof.
2. Description of the Related Art
Fluorocarboxylic acids, such as C8 fluorocarboxylic acids and their derivatives, are known to have good surface activity. These fluorocarboxylic acids, as well as their ammonium salts and alkali metal salts, are widely used as polymerization emulsifying agents in the polymerization of fluoroolefins such as tetrafluoroethylene. Moreover, it is well known that such fluorocarboxylic acids are generally prepared by hydrolyzing the corresponding fluorocarboxylic acid fluorides.
In one known process for preparing perfluorocarboxylic acids, the reaction product of fuming sulfuric acid with a perfluoroalkyl iodide is subjected to liquid-liquid separation between a light liquid phase and a heavy liquid phase, the perfluorocarboxylic acid fluoride present in the separated light liquid phase is hydrolyzed to give a perfluorocarboxylic acid-containing reaction product, and the resulting reaction product is subjected to distillation and separation, yielding a perfluorocarboxylic acid (JP-B 2-61446, U.S. Pat. No. 4,400,325).
In this related art process, many impurities form because the separated light liquid phase is directly hydrolyzed without distillation. Moreover, owing to the hydrogen fluoride (and therefore hydrofluoric acid) which forms as a result of the hydrolysis reaction between perfluorocarboxylic acid fluoride and water, the distillation apparatus used for distillation following hydrolysis either must be made of a corrosion-resistant precious metal material, or must be lined with a material such as a fluorinated polymer (e.g., polytetrafluoroethylene) or copolymer.
Thus, a process in which a perfluorocarboxylic acid fluoride-containing light liquid phase is directly hydrolyzed does not lend itself readily to industrial application for reasons having to do with the materials of the process equipment. Moreover, the reaction product subjected to distillation following hydrolysis contains various reaction by-products as impurities. Removal of the impurities increases the complexity of perfluorocarboxylic acid purification. Furthermore, because perfluorocarboxylic acid has a high boiling point, distillation must be carried out under a vacuum. Hence, the conditions for removal of the impurities and distillation make the purification process complex.
Another related art process, proposed as a solution to the above-described problems, is a process for preparing high-purity perfluorocarboxylic acid fluoride by the preliminary distillation of a light liquid phase containing perfluorocarboxylic acid fluoride (JP-A 8-231462). In this process, high-purity perfluorocarboxylic acid fluoride obtained by distillation is added to water and subjected to a hydrolysis reaction, yielding perfluorocarboxylic acid. Hydrogen fluoride forms as a by-product at this time. By carrying out hydrolysis at a lower temperature than the melting point of perfluorocarboxylic acid, the above-described problem of handling hydrogen fluoride in a distillation step can be avoided as described below, thus making this an advantageous process with industrial potential.
For example, when a perfluorocarboxylic acid having eight or more carbon atoms is prepared, the perfluorocarboxylic acid obtained by hydrolysis is a solid substance at room temperature. Because water adheres to the surface of this solid substance, dehydration must be carried out. The melting point of a perfluorocarboxylic acid having eight carbon atoms, for instance, is 53° C. The dehydration operation is generally performed using a centrifugal dehydrator. The hydrogen fluoride present within the aqueous phase is removed at the same time. This method requires handling a solid, but solids are less easily handled than liquids. Moreover, the amount of perfluorocarboxylic acid which dissolves in the aqueous phase that is separated by centrifugation and its loss cannot be disregarded. Also, removal of the hydrogen fluoride present in the adhering water may require several washes, further increasing the loss of perfluorocarboxylic acid. Finally, cases that involve the handling of a solid substance do not lend themselves well to a continuous process.
One way to overcome the above problems relating to the separation of a hydrogen fluoride-containing aqueous phase from solid perfluorocarboxylic acid is a process in which the perfluorocarboxylic acid is melted in the presence of the aqueous phase and liquid separation is induced based on the poor solubility of perfluorocarboxylic acid in water, thereby separating the contents into an aqueous phase and a perfluorocarboxylic acid phase. In this method, under temperature conditions at or above the melting point of the perfluorocarboxylic acid, the perfluorocarboxylic acid is rendered into a molten state and can be obtained by separation into a hydrogen fluoride-containing aqueous phase and a perfluorocarboxylic acid-containing organic phase.
However, when preparing a perfluorocarboxylic acid having eight carbon atoms, for example, under temperature conditions at or above the melting point of perfluorocarboxylic acid, the perfluorocarboxylic acid and water form a gel over a wide range in composition, making liquid-liquid separation difficult. “Gel,” as used herein, refers to a semisolid colloid having a three-dimensional network-like structure and lacking fluidity; that is, a physical gel. Hence, it is also not that easy to handle perfluorocarboxylic acid by melting it.